I have found that a kind of guanine nucleotide-binding protein (G-protein) is associated with the cytidylate cyclase system in rat brain and it functionally regulates the enzyme activity. This was confirmed by the significant increase of enzyme activity after treatment with G-protein-specific activators, AlF_4^- and GTPgammaS, and by the disappearance of GTPgammaS-induced activation in the presence of G-protein inactivator, GDPbetaS. However, the responsible G-protein in cytidylate cyclase system was apparently different from well-known G-proteins such as G_i and G_s in their susceptibility to ADP-ribosylation by bacterial toxins. The catalytic unit of this enzyme was partially purified through the activation by GTPgammaS, effective solubilization by CHAPS, gel filtration on Superose 6 and DEAE-cellulose column chromatography. This was the first report on its purification, though the purity was still 40-fold over its crude membrane fraction. This preparation had a very high molecular weight on gel filtration due to its aggregation, showing distinct chromatographic behavior from that of adenylate cyclase. In this study, androgen was found to be a possible endogenous factor regulating enzyme activity directly. Although the mechanism of activation is unknown, these evidence support the important role of cytidylate cyclase and its product, cCMP, in the brain and other tissues.